Single photon IR detectors and their integration with silicon detectors

ABSTRACT

Apparatuses and systems for photon detection can include a first optical sensing structure structured to absorb light at a first optical wavelength; and a second optical sensing structure engaged with the first optical sensing structure to allow optical communication between the first and the second optical sensing structures. The second optical sensing structure can be structured to absorb light at a second optical wavelength longer than the first optical wavelength and to emit light at the first optical wavelength which is absorbed by the first optical sensing structure. Apparatuses and systems can include a bandgap grading region.

PRIORITY CLAIM

This document claims the benefit of U.S. Provisional Application No.61/080,172 entitled “Integrated IR Detector” and filed on Jul. 11, 2008,which is incorporated by reference as part of the disclosure of thisdocument.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with government support under Grant No.W911NF-05-1-0243 awarded by ARO. The government has certain rights inthe invention.

BACKGROUND

This document relates to semiconductor devices including semiconductorphotodetectors and photodetector arrays.

Semiconductor pn junctions can be used to construct photodiodes fordetecting photons. An avalanche diode is one example of suchphotodiodes. Single-photon avalanche diodes (SPADs) are designed todetect single photons and can be used in a variety of applications,including biological, military and biometric applications. A SPAD can beoperated in a Geiger mode to output a signal that represents aphoton-arrival event or the time of arrival of a photon. As such, SPADscan be used as low-light-level imagers, sensors for Time-CorrelatedSingle-Photon Counting and Fluorescence Correlation Spectroscopy andother sensing applications. When operated in the Geiger mode, a SPAD isin essence a reverse-biased pn junction which can sustain numerousavalanche breakdowns without incurring damage and with minimal chargetrapping. When a photon is absorbed in the high-field region of theSPAD, the SPAD generates an electron-hole pair which can induce anavalanche through impact ionizations. This avalanche can be electricallysensed with high timing accuracy, and is quickly quenched. The pnjunction is then reactivated by recharging the junction in excess of itsbreakdown voltage.

SUMMARY

This document describes technologies, among other things, for singlephoton detection.

In one aspect, apparatuses and systems for photon detection can includea first optical sensing structure structured to absorb light at a firstoptical wavelength; and a second optical sensing structure engaged withthe first optical sensing structure to allow optical communicationbetween the first and the second optical sensing structures. The secondoptical sensing structure can be structured to absorb light at a secondoptical wavelength longer than the first optical wavelength and to emitlight at the first optical wavelength which is absorbed by the firstoptical sensing structure.

These and other implementations can include one or more of the followingfeatures. The first optical sensing structure can include a siliconsubstrate and is a silicon-based optical detector. The second opticalsensing structure can include an IR optical detector which emits lightat the first optical wavelength by converting energy in absorbed lightat the second optical wavelength via luminescence resulting from hotcarrier recombination. The first optical sensing structure can include asilicon detector. The second optical sensing structure can include aIII-V semiconductor stack. The second optical sensing structure caninclude an InGaAs/InP stack.

In some implementations, apparatuses and systems can include a silicondielectric layer as an interfacing layer formed between the firstoptical sensing structure and the second optical sensing structure.Apparatuses and systems can include a complementarymetal-oxide-semiconductor (CMOS) circuit to control the first opticalsensing structure and the second optical sensing structure.

In some implementations, the second optical sensing structure caninclude an absorption structure which absorbs light at the secondoptical wavelength and a multiplication material layer between theabsorption structure and the first optical sensing structure to emitlight at the first optical wavelength. The absorption structure can havea bandgap less than a bandgap of the multiplication structure. Theabsorption structure can have a bandgap similar to a bandgap of themultiplication structure.

In some implementations, the second optical sensing structure caninclude an absorption structure which absorbs light at the secondoptical wavelength and a multiplication structure between the absorptionstructure and the first optical sensing structure to emit light at thefirst optical wavelength. The absorption structure can have a bandgapless than a bandgap of the multiplication structure. The absorptionstructure can have a bandgap similar to a bandgap of the multiplicationstructure.

In some implementations, apparatuses and systems can include adielectric layer interfacing between the first and the second opticalsensing structures to permit transmission of light and to fuse the firstand the second optical sensing structures together as a singlestructure. The first and the second optical sensing structures can befused together as a single structure are free of an electrical contactstructure between the first and the second optical sensing structures.The first optical sensing structure can include a SiO₂shallow-trench-isolation (STI) guard ring structure.

The first optical sensing structure can include an array of detectorpixels. The second optical sensing structure can include an array ofdetector pixels. Implementations can include one or more layers tominimize inter-pixel cross-talk between the first and second opticalsensing structures.

The second optical sensing structure can include an absorption regionstructured to absorb photons at the second wavelength; a multiplicationregion structured to generate an avalanche of electrons in response toan absorbed photon to cause photons to be emitted at the first opticalwavelength; and a buffer region coupled with the multiplication region,and structured to impede electrons or holes from the avalanche frompassing through the buffer region to cause a reduction in an electricfield across the multiplication region to quench the avalanche, and toallow electrons or holes to pass through the buffer region to cause anincrease in the electric field across the multiplication region tofacilitate a recovery from the avalanche. Implementations can include abandgap grading region coupled with the multiplication region, at leasta portion of the bandgap grading region having a spatially varyingbandgap profile that monotonically changes between a first region thatinterfaces with the multiplication region and a second region.

In yet another aspect, apparatuses and systems can include asemiconductor absorption region structured to receive light at a firstwavelength to generate one or more charged carriers by absorbingreceived light; a multiplication region structured to receive the one ormore charged carriers generated from the semiconductor absorption regionand to generate an avalanche of secondary charged carriers in responseto the one or more charged carriers and emit secondary photons from thesecondary charged carriers; a region coupled between the absorptionregion and the multiplication region, the region comprising a mechanismthat quenches the avalanche of the multiplication region afteroccurrence of the avalanche and resets the multiplication region for anext avalanche; and a semiconductor transition region formed between themultiplication region and the absorption region to have a first bandgapat a first interface with the multiplication region that is equal to orsimilar to a bandgap of the multiplication region and a second bandgapat a second interface with the absorption region that is equal to orsimilar to a bandgap of the absorption region, the semiconductortransition region having a spatially varying bandgap between the firstand second interfaces to eliminate an abrupt change in bandgap betweenthe multiplication region and the absorption region.

In yet another aspect, apparatuses and systems can include asemiconductor absorption region structured to receive light at a firstwavelength to generate one or more charged carriers by absorbingreceived light; a multiplication region structured to receive the one ormore charged carriers generated from the semiconductor absorption regionand to generate an avalanche of secondary charged carriers in responseto the one or more charged carriers and emit secondary photons from thesecondary charged carriers; a region coupled between the absorptionregion and the multiplication region, the region comprising a mechanismthat quenches the avalanche of the multiplication region afteroccurrence of the avalanche and resets the multiplication region for anext avalanche; and a semiconductor transition region formed between thebuffer region and the absorption region to have a first bandgap at afirst interface with the buffer region that is equal to or similar to abandgap of the buffer region and a second bandgap at a second interfacewith the absorption region that is equal to or similar to a bandgap ofthe absorption region, the semiconductor transition region having aspatially varying bandgap between the first and second interfaces toeliminate an abrupt change in bandgap between the buffer region and theabsorption region.

In yet another aspect, apparatuses and systems can include asemiconductor absorption region structured to absorb photons at a firstwavelength to generate one or more charged carriers; a multiplicationregion structured to receive the one or more charged carriers, themultiplication region structured to generate an avalanche of electronsin response to the one or more charged carriers and emit secondaryphotons at a second wavelength shorter than the first wavelength; abuffer region structured to impede electrons or holes from the avalanchefrom passing through the buffer region to cause a reduction in anelectric field across the multiplication region to quench the avalanche;and a bandgap grading region adjacent to the absorption region, at leasta portion of the bandgap grading region having a spatially varyingbandgap profile that monotonically changes between a first region thatinterfaces with the absorption region and a second region.

These and other implementations can include one or more of the followingfeatures. In some implementations, the buffer region can be structuredto allow electrons to pass through the buffer region to cause anincrease in the electric field across the multiplication region tofacilitate a recovery from the avalanche. In some implementations, thebuffer region can be structured to allow holes to pass through thebuffer region to cause an increase in the electric field across themultiplication region to facilitate a recovery from the avalanche. Insome implementations, the bandgap grading region can be positionedbetween the absorption region and the multiplication region, wherein thesecond region of the bandgap grading region interfaces with themultiplication region. In some implementations, the bandgap gradingregion can be positioned between the absorption region and the bufferregion, wherein the second region of the bandgap grading regioninterfaces with the buffer region. The buffer region and themultiplication region can be structured to create an energy barrier by avalence band offset. Implementations can include a mechanism to bias thesecond optical sensing structure at a DC voltage. The multiplicationregion can be optically coupled with an optical sensing structure. Thebuffer region can include an InAlAs layer. The buffer region can includean InAlAs, InGaAsP, and InAlAs stack. The bandgap grading region caninclude one or more InGaAsP graded index layers.

The details of one or more implementations are set forth in theaccompanying drawings, and the description and the claims in thisdocument.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of an InP energy band diagram that illustrateshot-carrier luminescence in a direct recombination processes.

FIG. 2 shows an example of a calculated spectrum of electroluminescentphotons emitted at the junction of an InP pn junction.

FIG. 3 shows a cross-section example of a device based on a hot-carrierluminescence effect.

FIG. 4 shows an example of a geometrical construction for calculatingthe percentage of photons emitted from an InP junction plane onto a Sijunction plane.

FIG. 5 shows a graph based on a numerical analysis example of electron,hole, and total avalanche initiation probabilities as a function ofphoton absorption depth in a Si SPAD.

FIG. 6 shows an example of junction and surface electroluminescencespectral densities for a 200 nm deep InP junction.

FIG. 7 shows an example of a numerical simulation of an internalupconversion efficiency as a function of primary SPAD's junctioncapacitance.

FIG. 8A shows an example of electric field distributions associated witha structure that includes a diffused guard ring structure.

FIG. 8B shows an example of electric field distributions associated witha structure that includes a STI guard-ring structure.

FIGS. 9A, 9B, and 9C show instrument response function examples fordifferent STI-bound SPAD devices.

FIG. 10A shows an example of a device that includes a SPAD layout forself-quenching and self-recovery.

FIG. 10B shows example of a band diagram during self quenching and selfrecovery.

FIG. 11A, 11B, 11C show an example of a sequence of events in a detectordevice that includes a Transit Carrier Buffer.

FIGS. 12A and 12B show different response profile examples of aself-quenching, self-recovery SPAD.

FIGS. 13A, 13B, 13C, 13D show different graphs associated with aself-quenching, self-recovery SPAD.

FIGS. 14A, 14B, 14C, and 14D show response examples of a self-quenching,self-recovering SPAD to multiple input photons at various rates.

FIG. 15A shows a graph of a relationship between Geiger-mode gain andhole escape time.

FIG. 15B shows an example of a relationship between Geiger-mode gain andTCB layer thickness.

FIG. 16 shows example of an I-V response of a self-quenching andself-recovering device in the dark and in 1550 nm illumination.

FIG. 17A shows an example of an output pulse height at a bias voltage of30.4 V.

FIG. 17B shows an example of output pulse signals triggered by a seriesof single photons.

FIG. 18 show an example of different single photon detectionefficiencies versus dark current rates at various temperatures.

FIG. 19 shows an example of a recovery time profile.

FIG. 20 shows a cross-section example of a SPAD in a detector device.

FIGS. 21A and 21B show SPAD devices having a semiconductor transitionregion between the multiplication region and the secondary photoabsorption region.

FIGS. 21C and 21D show different bandgap grading region layouts.

FIGS. 22A, and 22B show different implementation details of anInGaAs/InAlAs SPAD with a built-in quenching mechanism.

FIG. 23 shows a different cross-section example of a device based on ahot-carrier luminescence effect.

FIG. 24 shows an example of a detector architecture.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This document describes, in one aspect, examples of single photondetection apparatuses and systems such as integrated infrared (IR)detectors and detector arrays based on a hybrid structure comprised oftwo avalanche detectors for detecting light at two different wavelengthsthat are coupled to each other in an integrated package. For example, anIR avalanche diode can be coupled to a silicon single-photon avalanchedetector to form this hybrid structure. Such a device can be designed toproduce electroluminescence resulting from hot-carrier recombination asa upconversion mechanism to convert received light into light of ashorter wavelength. The two detectors can be two SPADs.

In some implementations of the hybrid structure, an interconnectionbetween an IR avalanche diode and the silicon avalanche diode isexclusively optically, and thus no electrical interconnection isrequired between the IR avalanche diode and the silicon avalanche diode.This optical interconnection makes the manufacturing of the devicessimple and inexpensive because the two detectors formed on two differentsubstrates can be coupled to each other by fusing their dielectricpassivation layers without requiring lattice matching between the twodevices. Advantageously, the hybrid structure can be made for various IRdetecting materials at various IR wavelengths for the IR detectorwithout limitations imposed by the lattice matching between the twodevices. Because of lack of electrical interconnection, electricallyconductive bumps for electrical interconnection are not needed and thusdetector pixels in a detector array can be packed more closely together,making it possible to achieve small pixel pitch for high-resolution IRimaging.

The above hybrid structure of an IR detector and a Si detector is anexample of a detection device or an array element that includes aprimary detector such as an IR detector and a secondary detector thatdetects light emitted by the primary detector. The IR detector (theprimary detector) may be formed of III-V materials (e.g., InGaAs/InP)and other suitable IR absorbing semiconductor materials. The IR detectormay be structured to include a separate absorption layer and amultiplication layer in a separate absorption and multiplication (SAM)structure. The secondary detector, e.g., a silicon detector such as anshallow-trench-isolation (STI)-bounded Si detector or a suitabledetector, can be based on a CMOS process, a BiCMOS process or othersuitable silicon process.

In some implementations of the SAM structure, an emission can beincreased by introducing an energy discontinuity at the edge of themultiplication region, e.g., away from the absorption layer. Thisdiscontinuity causes a local accumulation of the hot carriers, therebyincreasing the probability of recombination and thus theelectroluminescence yield that can be measured by the number of photonsemitted per hot carrier. As an example, a semiconductor device based onthe technical features in this document can include a substrate; a firstoptical sensing structure formed on the substrate to absorb light at afirst optical wavelength; and a second optical sensing structure formedon a second substrate and fused over the first optical sensingstructure. The second optical sensing structure can be structured toabsorb light at a second optical wavelength longer than the firstoptical wavelength and to emit light at the first optical wavelengthwhich is absorbed by the first optical sensing structure. The first andsecond optical wavelengths are different optical wavelength bands, e.g.,a silicon detector can be used to detect light from 400 nm to 1100 nm.In one implementation of this device, the substrate is a siliconsubstrate, the first optical sensing structure is a silicon-basedoptical detector, and the second optical sensing structure is an IRoptical detector which emits light at the first optical wavelength byconverting energy in absorbed light at the second optical wavelength vialuminescence resulting from hot carrier recombination. The secondoptical sensing structure can include an absorption structure whichabsorbs light at the second optical wavelength and a multiplicationmaterial layer between the absorption structure and the first opticalsensing structure to emit light at the first optical wavelength. Theabsorption structure can have a bandgap less than a bandgap of themultiplication structure.

In implementations of detector devices based on the above hybridstructure, a detector device can include an IR avalanche detector and anSi detector that are fused through a silicon dioxide layer. The IRavalanche photodiode can be operated either in sub-Geiger or in Geigermode as a single-photon detector. For example, the IR single photonavalanche photodiode (SPAD) can be constructed with a separateabsorption and multiplication (SAM) zones. As a specific example, theabsorption zone can be in InGaAs with a narrow bandgap, which canefficiently absorb photons, e.g., at 1550 nm which is used incommunications and other applications, and the multiplication zone canbe in InP, a direct bandgap material with a bandgap of 1.4 eV which ishigher than that of silicon. A photogenerated electron-hole pair caninduce an avalanche by a chain reaction of impact ionizations in the InPregion. The accelerated (hot) electrons can impact stationary holes andthe recombination of the hot electrons and the holes releases the excessenergy with a known distribution that has a component above the energybandgap of the material (InP). Therefore, this component manifestsitself as a secondary radiation of electroluminescent photons and can bedetected in silicon. The electroluminescent photons can be emittedisotropically. Some of such electroluminescent photons can reach thedepletion region of the silicon SPAD, which is biased above itsbreakdown voltage. As a result, an avalanche breakdown occurs, and isdetected by sensing circuitry, either on or off the same silicon die.

The IR and visible-wavelength SPADs in hybrid structures can bestructured and operated in various configurations. For example, the IRSPAD can be configured to be back-illuminated so the emission occurs atthe side facing the Si SPAD, or illumination can be through the backside of the silicon SPAD (which is transparent to IR wavelengths) andthe primary photons in IR impinge directly on the front surface of theIR detector. The IR SPAD can have a multiplication region that includesa direct-bandgap material such as InP or GaAs, which has a higherphoton-to-hot-electrons electroluminescence yield than indirect-bandgapmaterials. The IR SPAD can be configured to have its quenching mechanismcontained within the device itself to maximize the performance gain fromthis scheme. The IR SPAD can have as shallow a junction as possibleunderneath the surface to minimize self-absorption of the secondaryphotons, and should have as thin a passivation layer as possible, tominimize dispersion of these photons away from the silicon SPAD. Thesilicon SPAD can be designed to have as wide an absorption region aspossible, in order to increase the absorption probability of theelectroluminescent photons. The two SPADs can be fused at the silicondioxide layer or a suitable common passivation layer with minimal voidsto reduce optical loss and enhance the mechanical integrity. The fusingcan be performed either on wafer level or per die. The former can offersignificant cost saving.

In addition, structures may be included in the two SPADs to enhance ormaximize coupling between the two SPADs or to reduce inter-pixelcross-talk in an array of sensors. For example, such structures caninclude micro-lenses between the two SPAD detectors for collecting andfocusing emitted light from the IR SPAD onto the silicon junction of thesilicon SPAD; trenches between IR pixels to reduce optical cross-talk;metal rings around the silicon or IR pixels to prevent photons emittedfrom one pixel to reach adjacent pixels; and dielectric structuresaround the IR devices to reflect photons that are emitted away from thesilicon device back towards the silicon device for efficient collectionand detection of photons at the silicon device.

In some implementations, IR detectors tend to exhibit high dark countswhich compromise the IR detection performance. Techniques are providedto significantly reduce or virtually eliminate the effect on the IRdetection performance caused by the dark current counts. In cases whereactive illumination is used by, e.g., using a laser beam to illuminate atarget under imaging, an electrical signal correlated to the laser pulseeither biases the IR detector such that the IR detector is only activein a limited time window, or does the same to the silicon SPAD, oractivates a read-out circuit such that signals are only read in a timewindow when signals are expected. Another scheme for eliminating theeffect of dark counts can be used when the resolution of the image isless than the resolution of the IR array. In such a scenario, a “real”illumination will cause an avalanche in at least two adjacent IR pixels,which will be detected by a corresponding number of silicon pixels. Alogic gate can locally process the SPAD outputs and only pass thosepulses which are valid (dark counts are non-correlated so theprobability of two adjacent cells firing simultaneously due to darkcounts can be shown to be negligible for realistic sampling time-gates).

Such an integrated detector structure can be used to provide scalabilityfor forming large arrays such as processing parallel communicationchannels simultaneously and array imaging (e.g., 3D imaging), as well asfor high-sensitivity infrared imaging. The pitch of detection pixels inthis case is not limited by present wafer-level interconnection schemessuch as indium bumps or cross-vias because optical interconnection isused. Optical and/or physical isolation structures may be used, such asmesa structures, for limiting the avalanche propagation or metalbarriers to provide optical isolation between adjacent pixels. In someimplementations, an electrical readout can be eliminated for a primarydetector to reduce the junction capacitance of an IR detector, because asecondary detector can process optical information coming from theprimary detector. The described technologies can produce significantreductions in power consumption and other performance improvements suchas improved speed and reduced afterpulsing.

Several techniques can be used to evaluate single-photon detectors. Thesingle-photon detection probability, η, is the product of theprobabilities of a photon being absorbed in the material and of itinitiating a detectable avalanche. The detector's spectral responsedescribes the wavelength-dependence of this detection probability. Thesemetrics depend on the percentage of pixel area which collect photons(fill ratio); on the absorbing layer's composition, depth and thickness;and on the electric field distribution in the multiplication region.

During the recharge process following an avalanche, the SPAD can betemporarily biased below the breakdown voltage, and cannot generate anavalanche pulse in response to a photon. This time is called the devicedead time and depends on the recharge mechanism, on the overbias abovebreakdown and, most significantly, on the junction's capacitance.

Dark counts result from avalanches which are not induced by absorbedphotons. They can originate from thermally-generated carriers; fromband-to-band tunneling; via trap-assisted tunneling; and byafterpulsing—the release of carriers trapped in prior avalanches. Thelatter mechanism is an important factor in determining the device deadtime.

The time-delay spread between the photon absorption event and theclocking of the resulting electrical signal depends on the diameter ofthe SPAD as well as on the timing circuitry. The time-delay spread candetermine the timing resolution of the single-photon detector.

Solid-state IR SPADs detectors can have separate absorption andmultiplication regions, whereby, for example, photons are absorbed in astructure such as a thick (e.g., several microns) lightly-doped InGaAslayer. The photo-generated carriers are swept towards an InP high-fieldmultiplication region where impact ionizations provide gain. When theextraction rate of carriers from this multiplication region falls belowthe creation rate, an avalanche breakdown is said to occur and the gainbecomes “infinite”. Some detectors can operate in a Geiger Mode (GM) forsingle-photon detection. The avalanche must be quenched to avoid damageto the junction.

In some implementations, quenching can be achieved passively, by using avoltage-limiting resistor in series with the device, or actively, usinga circuit which senses the onset of the avalanche, and subsequentlyquenches it. Once the avalanche has been quenched, the diode capacitancecan be re-charged, either passively, through the quenching resistor, oractively, using a recharging circuit. In some implementations, a SPADcan be self-quenched.

IR Geiger-mode single photon avalanche diodes (GM-SPADs) can be amenableto integration in large arrays and to mass production because they canbe manufactured using standard lithographically-defined processingtechniques. However, the support circuitry, including the quenching,recharging and processing circuitry must be implemented externally,usually in silicon.

Further, GM-SPADs may experience excessive amounts of noise.Hole-electron pairs, thermally generated at the edge of the high fieldregion through Shockley-Read-Hall generation and separated by the strongelectric field, can cause a “false” avalanche. Trap-assisted tunnelingdepends on the defect density in addition to the doping and may beexacerbated at high electric fields by barrier lowering via thePoole-Frenkel effect. Direct band-to-band tunneling requires strongelectric fields above 7×105 V/cm occurs in devices with a breakdownvoltage lower than 4E_(G)/q, where EG is the bandgap energy and q is theelectron charge. Finally, afterpulsing which results from the release ofdeep traps trapped during previous SPAD cycles, increases with highdefect densities and is linearly dependent on the total charge flowingduring an avalanche. The rate of emission of deep traps follows anexponentially decaying distribution which depends on the activationenergy of each deep trap mechanism.

Some of these noise sources can be reduced by cooling. Deep traplifetimes can increase exponentially as temperature is decreased.Thermal generation and tunneling increase as the bandgap decreases.Thus, in some implementations, cooling an IR SPAD may be required. Forexample, some IR SPAD may be cooled to about 200° K. At thesetemperatures, afterpulsing becomes the dominant noise source with rateson the order of tens of kHz and it becomes the main bottleneck fordevice bandwidth. Time-gating can reduce the effect of afterpulsing butdue to its exponential time distribution, the separation between gates(device dead time) must be made long enough compared with the afterpulselifetime, in order to sufficiently reduce the probability ofexperiencing an afterpulse during an exposure time gate. Furthermore,time gating is possible when the arrival time of the photon is known towithin the duration of the gate. At low temperatures, SPADs can beoperated in free-running mode with minimal afterpulsing effects where asufficiently long hold-off time is used following an avalanche, therebyseverely limiting the detection rate.

For a given technology, afterpulsing can be reduced by limiting thecharge flowing during an avalanche. This may be done by active quenchingbut is achieved more efficiently by reducing the junction capacitance.The capacitance in IR SPADs is dominated by the capacitances of thereadout, recharge and quenching circuitry, because these operations areimplemented off-chip, either on a board or on a silicon die. Connectionscan be made using technologies such as wire bonding or by using indiumbumps. This can limit the pixel pitch and may result in capacitances onthe order of pF, thus deteriorating the noise performance of the devicedue to afterpulsing.

This document includes a description of integrated detectors thatinclude a new interconnection and readout scheme which does not requireelectrical interconnections between an IR SPAD and a CMOS readoutcircuitry, offers superior upconversion efficiencies with low power, andis scalable to large arrays. By bypassing the requirement of electricalbonding between the SPAD pixels and the readout circuitry, thecapacitance seen by the junction is significantly reduced. This resultsin a reduction in afterpulsing, which is the dominant noise source atlow temperatures, while simultaneously decreasing the detector device'sdead time.

An integrated detector device can be based on wavelength upconversionusing a byproduct of the avalanche process, e.g., hot-carrierluminescence from a multiplication layer of the device. Thisluminescence can have a significant component at higher energies thanthe bandgap of the absorbing material, and can therefore be extended toseveral IR detection materials. Readout of the luminescent photons canbe achieved by a coupled detector such as a silicon single-photonavalanche diode.

A manufacturing process for an integrated detector device can includeutilizing a mature wafer-level glass-to-glass fusing technology in orderto connect different components of an integrated detector device (see,e.g., J. Wei, S. M. L. Nai, C. K. S. Wong, Z. Sun, and L. C. Lee, “Lowtemperature glass-to-glass wafer bonding,” IEEE Transactions on AdvancedPackaging, vol. 26, pp. 289-294, 2003). For example, a wafer-levelglass-to-glass fusing technology can form a connection between a III-VSPAD which detects the primary IR photon, and a silicon CMOS SPAD, whichdetects the up-converted photons, and which processes the information onthe same die.

Hot-carrier luminescence can result from a recombination of hotelectrons with holes. In direct bandgap materials, a photon is emittedwhose energy equals the difference between the hot electron's initialenergy and the bandgap. In an indirect recombination process, bothenergy and momentum must be exchanged, and the probability of suchevents is considerably lower, depending on the availability of suitablephonons. This is manifested by the different electron temperatures inthese processes. In GaAs, the direct recombination process ischaracterized by an electron temperature of 800K while the indirectprocess in the same material exhibits a temperature of 3000K. The highertemperature stems from a longer mean lifetime, consistent with a lessprobable recombination event.

FIG. 1 shows an example of an InP energy band diagram that illustrateshot-carrier luminescence in a direct recombination processes. A hotelectron accelerated by the strong electric field recombines with a holeat the valence band. The excess energy is released in the form of aphoton with energy

ω>E_(gd). A low-energy, infrared, component due to transitions betweenthe light and heavy hole bands is also observed.

For a direct bandgap material, such as GaAs or InP, the photon emissionrate is given by:R _(d)(

ω)∂

ω(

ω−E _(gd))^(1/2) f(E)[1−f(E−

ω)]  (1)where

ω is the emitted photons' energy, E_(gd) is the direct bandgap; E is theelectron energy above the bottom of the conduction band; f(E) and[1−f(E−

ω)] are the hot electron and the hole distributions, respectively, bothof which strongly depend on the hot-carrier temperature.

FIG. 2 shows an example of a calculated spectrum of electroluminescentphotons emitted at the junction of an InP pn junction. The emissionspectrum of InP was calculated using equation (1), and is shown in FIG.2. Because the photons carry the excess energy after the recombination,

ω>E_(gd) thereby achieving energy upconversion.

The efficiency of this upconversion depends on the electroluminescenceyield, e.g., the photon emission rate per unit avalanche charge. Thisfigure can be difficult to measure due to self-absorption by theemitting device, the detector's spectral response, the effect ofdefects, and collection uncertainties due to reflections. Kurtsiefer etal reported a figure of 39 photons per steradian in an avalanche with4×10⁸ electrons, resulting in a lower limit of 2.5×10⁻⁶ photons perelectron, where the detector's spectral response has been partlyaccounted for, and self-absorption was not (C. Kurtsiefer, P. Zarda, S.Mayer, and H. Weinfurter, “The breakdown flash of silicon avalanchephotodiodes-back door for eavesdropper attacks?,” Journal of ModernOptics, vol. 48, pp. 2039-2047, 2001). A measurement accounting for boththe optical system and self-absorption was presented by Lacaita, with anemission efficiency of 2.9×10⁻⁵ photons with energy higher than 1.14 eVper carrier crossing the junction (A. L. Lacaita, F. Zappa, S.Bigliardi, and M. Manfredi, “On the Bremsstrahlung origin ofhot-carrier-induced photons in silicon devices,” IEEE Trans. ElectronDev., vol. 40, pp. 577-582, 1993). Electroluminescence yield for InP hasnot been reported to date. It is expected to be significantly higherthan that of silicon and is conservatively estimated to be 2.9×10⁻⁴photons per hot-carrier for the purposes of our calculation.

FIG. 3 shows a cross-section examples of a device based on a hot-carrierluminescence effect. Such a device can include an InGaAs/InP SPAD fusedwith a silicon CMOS SPAD. In some implementations, the InGaAs/InP SPADcan include electrodes such as Al electrodes positioned on a layer ofthe InGaAs/InP SPAD. The InGaAs/InP SPAD can include an InP substrate,an InGaAs absorption layer, an InGaAsP grading, and a multiplication andemission region. The multiplication and emission region can include ann-InP charge layer, an i-InP layer, and a p+ InP layer. In someimplementations, a primary IR photon is absorbed in a narrow-bandgapInGaAs layer, and the photogenerated charges are swept to a high-fieldmultiplication region. During avalanche multiplication, secondaryphotons are emitted from the multiplication layers and are detected bythe silicon SPAD, and processed on the same die. This upconvertinghybrid pixel can be scaled to large arrays for parallel operation, forexample in single-photon near infrared (NIR) imaging applications. Ametal masking layer is used to minimize inter-pixel cross-talk. In someimplementations, the silicon SPAD can include one or more Al regionspositioned in an oxide. The silicon SPAD can include ashallow-trench-isolation (STI) guard ring structure. The silicon SPADcan include p+ regions, n+ regions, a p+ implant, and a N-well regionpositioned on a Si p-substrate.

In some implementations, an InGaAs SPAD is direct-bonded to a Si SPADfor optical readout. IR photons are incident on the back surface of theInGaAs SPAD. As carriers recombine during the avalanche, they releaseNIR and visible photons which are detected by the silicon device. Thedevices can be fused through their silicon-dioxide passivation layer,eliminating the need for lattice matching between the two semiconductingmaterials.

Determining the overall detection probability of the up-conversionscheme can include multiplying the primary NIR detection probability inthe InGaAs/InP SPAD, by the emission probabilities (as a function ofwavelength) of the electroluminescent photons emitted towards thesilicon SPAD. One can account for self-absorption in the InP device. Onecan determine the probability of absorption of these photons in thesilicon SPAD's depletion region, and can calculate the avalancheinitiation probability of the photogenerated charge carriers. In someimplementations, an up conversion probability can be expressed as:

$\begin{matrix}{{\eta_{uc} = {\int_{E_{gs}}^{E_{\max}}{\int_{x_{j\; 2}}^{x_{j\; 2} + x_{d}}{\frac{\Omega}{4\pi}\ {{N_{sp}({\hslash\omega})}\left\lbrack {1 - {P_{sa}\left( {{\hslash\omega},x_{j\; 1}} \right)}} \right\rbrack} \times \times {P_{abs}\left( {{\hslash\omega},x_{abs}} \right)}{P_{av}\left( x_{abs} \right)}{\mathbb{d}({\hslash\omega})}{\mathbb{d}x_{abs}}}}}}\ } & (2)\end{matrix}$where Ω is the solid angle subtended by the silicon junction when seenfrom the InP junction; N_(sp) is the number of electroluminescentphotons at energies,

ω, emitted in a primary avalanche; P_(sa)(

ω,x_(j1)) is the probability of self-absorption of the secondaryphotons, which is a function of their energy and generation depthx_(j1); P_(abs) is the absorption probability in the silicon SPAD'sdepletion region, which extends from x_(j2) to x_(j2)+x_(d); and P_(av),(x_(abs)) is the probability for an electron-hole pair photo-generatedat x_(abs) to induce a detectable avalanche.

Operations of an integrated IR detector device can cause a secondaryphoton emission towards a silicon junction in response to one or morephotons hitting the primary detector of the device. An avalanche eventin a SPAD can be viewed as a discharge of the junction capacitance,C_(j), from an initial voltage, in excess of the diode's breakdownvoltage to approximately the breakdown voltage. The total number ofelectrons flowing during an avalanche is:

$\begin{matrix}{N_{e} = {\frac{1}{q}\left( {C_{j} + C_{p}} \right)V_{ob}}} & (3)\end{matrix}$where q is the electron charge (in Coulomb) C_(j) is the junctioncapacitance, C_(p) is any additional capacitance seen by the junction,including interconnection and sensing capacitances and V_(ob) is theoverbias above breakdown.

The number of secondary photons emitted from the primary junction can beexpressed as:N _(sp)(

ω)=η_(e) s(

ω)N _(e)  (4)where η_(e) is the luminescence yield per electron (in relevant energiesfor Si absorption) and s(

ω) is the normalized spectral distribution of the secondary photons. Insome implementations, additional photons can be expected to be emittedfrom the charge region between the absorption and multiplicationregions, where the high electric field is lower than the breakdownfield, and thus recombination events are highly likely. It can beassumed that secondary photons are emitted from the maximum fieldregion, at the junction plane.

FIG. 4 shows an example of a geometrical construction for calculatingthe percentage of photons emitted from an InP junction plane onto a Sijunction plane. A conservative approximation places all avalanchingcharges at one of the vertices of the emitting plane. It can be assumedthat there is an isotropic emission from the junction plane.Consequently, only a fraction of the emitted photons,

$\frac{\Omega}{4\pi},$is transmitted towards the silicon junction. The solid angle, Ω, for thecase of a planar InP junction emitting towards a parallel planar siliconjunction can be approximated by assuming all photons are emitted fromone of the vertices of the InP rectangular junction, e.g., FIG. 4. Thiscan provide a lower bound on the actual flux emitted towards the siliconSPAD. The solid angle subtended by the detector from this vertex can beapproximated by:

$\begin{matrix}{\Omega = {\tan^{- 1}\frac{d}{S\sqrt{{2d^{2}} + S^{2}}}}} & (5)\end{matrix}$where d is the side dimension of the Si junction and S is the verticaldistance between the junctions. Equations (3), (4), and (5) can becombined to give:

$\begin{matrix}{{\frac{\Omega}{4\pi}{N_{sp}(\lambda)}} = {\frac{1}{4\pi\; q}\tan^{- 1}\frac{d^{2}}{S\sqrt{{2d^{2}} + S^{2}}}\eta_{e}{S(\lambda)}\left( {C_{j} + C_{p}} \right)V_{ob}}} & (6)\end{matrix}$

Self-absorption in InP reduces the number of photons which reach thesurface. It can be assumed that luminescence occurs at the junctionplane, a distance x_(j1) from the surface. The emitted photon populationcan be expressed as:

$\begin{matrix}\begin{matrix}{{N_{surf}\left( {\hslash\;\omega} \right)} = {{\frac{\Omega}{4\pi}{{N_{s\; p}\left( {\hslash\;\omega} \right)}\left\lbrack {1 - {P_{sa}\left( {{\hslash\;\omega},x_{j\; 1}} \right)}} \right\rbrack}} =}} \\{= \frac{\left\lbrack {\tan^{- 1}\frac{d^{2}}{\sqrt{{2d^{2}} + S^{2}}}\eta_{e}{s\left( {\hslash\;\omega} \right)}\left( {C_{j} + C_{p}} \right)V_{ob}} \right\rbrack{\exp\begin{pmatrix}{- {\alpha_{InP}({\hslash\omega})}} \\x_{j\; 1}\end{pmatrix}}}{4\pi\; q}}\end{matrix} & (7)\end{matrix}$where α_(InP) (

ω) is an absorption coefficient in InP.

Based on a calculated spectral distribution of the emitted photons, onecan estimate the probability for these photons to be absorbed by the SiSPAD, and can estimate the probability of generating an avalanche. Itcan be assumed that reflections at the interfaces do not substantiallyaffect the number and spectral distribution of secondary photons.

In an STI-bounded shallow junction, the high field region can be highlylocalized in the depletion region of the junction, so it can be assumedthat absorption occurs within the depletion region. The probability forN photons of energy

ω to generate an electron-hole pair within this layer is:P _(abs) ^(N)(

ω)=1−{1−P _(abs)(exp[−α_(Si)(

ω)·w _(d)]−exp[−α_(Si)(

ω)·(x _(j2) +w _(d))])}^(N)  (8)with α_(Si) being the absorption coefficient in silicon, w_(d) thedepletion width and x_(j2) the junction depth in the silicon device. Thedepletion width of the junction can be determined from the analyticalexpression for a one-sided linearly-graded junction:

$\begin{matrix}{w_{d} = \left( \frac{3V_{B}ɛ_{s}}{2{qa}} \right)^{1/3}} & (9)\end{matrix}$where V_(B) is the sum of applied and built-in voltages, ∈_(s) is thedielectric constant of silicon, q the electron charge and a the gradingcoefficient of the linearly-graded junction. The total upconvertedphotons' absorption probability in silicon can be calculated usingequations (7), (8), and (9) over relevant wavelengths.

The probability that an absorbed photon induces an avalanche can becalculated. For a one-sided, linearly-graded pn junction, Poisson'sequation translates to a field distribution:

$\begin{matrix}{{E(z)} = {\frac{qa}{2ɛ_{s}}\left( {w_{d}^{2} - z^{2}} \right)}} & (10)\end{matrix}$The avalanche probability can be estimated as a function of the positionof generation of the electron-hole pair by solving the following coupleddifferential equations:

$\begin{matrix}{\frac{\mathbb{d}P_{be}}{\mathbb{d}z} = {\left( {1 - P_{be}} \right)\alpha\; P_{bp}}} & \left( {11a} \right) \\{\frac{\mathbb{d}P_{bh}}{\mathbb{d}z} = {\left( {1 - P_{bh}} \right)\beta\; P_{bp}}} & \left( {11b} \right)\end{matrix}$where P_(be) and P_(bh) are the avalanche initiation probability by anelectron and a hole, respectively, α and β are the ionization rates ofelectrons and holes, respectively, and P_(bp) is the joint avalancheinitiation probability:P _(bp)=1−(1−P _(be))(1−P _(bh))=P _(be) +P _(bh) −P _(be) P _(bh)  (12)These equations can be solved numerically to provide an avalanchingprobability.

FIG. 5 shows a graph based on a numerical analysis example of electron,hole, and total avalanche initiation probabilities (P_(e), P_(h) andP_(p)) as a function of photon absorption depth in a Si SPAD, based onequations (11a,b)-(12).

In some implementations, a detector device can include a self-quenchedInGaAs/InP SPAD with an active area of 15 μm per side and can have ajunction capacitance of 150 fF, dominated by the capacitance of thedepleted region. Due to the optical readout, off-chip routing and thesensing circuit's capacitance, which can be on the order of a picofaradin SPADs with electrical readout, are eliminated. In someimplementations, the passivation thickness of a 6-metal layer silicondevice is on the order of 7 μm, so InP-SPAD/Si-SPAD capacitance can benegligible. In some implementations, a self-quenched InGaAs/InP SPAD canoperate at an overbias of 5V with a junction located 200 nm below thesurface.

FIG. 6 shows an example of junction and surface electroluminescencespectral densities for a 200 nm deep InP junction as described herein.The latter accounts for only those photons emitted towards the siliconjunction. An estimate that 4.7×10⁶ electrons flow during an avalanchecan be attained from equation (3). An estimate that 131 photons areemitted isotropically in the silicon absorption band from the junctioncan be attained from equation (4). The spectral density of these photonsis shown in FIG. 6, both at the junction and, using equation (7), at thesurface of the InGaAs/InP SPAD.

FIG. 7 shows an example of a numerical simulation of an internal upconversion efficiency as a function of primary SPAD's junctioncapacitance. The up conversion efficiency can be determined using theemitted spectral density and the sensitivity of the silicon detector. Insome implementations, the total charge flowing in an InGaAs/InP SPAD canbe increased.

A detector device, such as a device with primary and secondarydetectors, can include a mechanism to prevent a positive feedback loopbetween the detectors. A silicon SPAD, positioned as a secondarydetector, can emit electroluminescent photons which can be absorded byan IR SPAD, positioned as a primary detector. Controlling the dead timeof the IR SPAD to overlap the avalanche time of a silicon SPAD canprevent a positive feedback loop between the coupled SPADs.

In some implementations, the power dissipated during an up conversionprocess is the sum of the powers dissipated during the InP and siliconavalanche. These can be estimated as the product of the junctioncapacitances by their overbias, resulting in approximately 1 pW perdetected photon.

Various examples of photon based systems that include one or moredetectors can include systems for quantum key distribution (QKD),quantum communications, pulse position modulation communication links,fluorescence lifetime imaging microscopy (FLIM), and time-correlatedsingle-photon counting (TCSPC), eye-safe laser detection and ranging(LIDAR), optical time-domain reflectometry (OTDR) and semiconductorfailure analysis systems. In some implementations, single-photondetectors can operate at high frequencies such as tens to hundreds ofMHz, can consume minimal power (<1 nW/bit), and can operate reliably atnon-cryogenic temperatures over many cycles and can be manufactured at alow cost. When operated in arrays, such detector devices can have asmall pitch and low pixel-to-pixel cross-talk. Further, such systems canfeature low detector jitter for more efficient operations. For example,lower detector jitter can result in lower bit-error-rates in quantum keydistribution systems, faster bit-rates in pulse position modulationoptical links, and better temporal resolution in biologicalapplications.

A silicon SPAD, such as a CMOS SPAD, can include a diffused guard-ringstructure which can lead to increased jitter due to the lateral driftand diffusion of electron-hole pairs created by photon absorption in thelow-field and neutral regions. The resulting diffusion tail can limitSPAD timing performance, e.g., in QKD systems and in classical pulseposition modulation optical links, where diffusion tails limit bit errorrates and bit rates, respectively, and in high-resolution FLIM, wherebetter timing resolution translates into better image contrast.Therefore, in some implementations, a silicon SPAD can feature a SPADarchitecture optimized not only for a minimal FWHM but also for aminimal full-width at hundredth-maximum (FW( 1/100)M).

FIG. 8A shows an example of electric field distributions associated witha structure that includes a diffused guard ring structure. In FIG. 8A,the electric field is given in volts per centimeter and coordinatemicrometers. Some detectors can use a diffused guard ring structure. Forexample, a detector can include a triple-well diffused guard ringstructure that has different electric field regions such as a high-fieldregion, a low-field (diffused) region, and a neutral region where thereis little or no electric field. Charges photogenerated inside thehigh-field region can induce an avalanche pulse almost instantaneouslythrough a chain reaction of impact ionizations. Carriers generated inthe low-field region can drift laterally toward the high-field regionand subsequently initiate impact ionizations. In addition, carriersgenerated in the neutral region can diffuse toward the high- orlow-field region and subsequently initiate an avalanche. The delay fromthe photogeneration event to the initiation of the avalanche that canincreases the uncertainty in the photon arrival time. Such a diffusedguard ring structure can result in a low-field region and, consequently,in increased timing uncertainties

A silicon SPAD, such as a CMOS SPAD, that has a low detector jittersuitable for implementations of a detection device that integrates asilicon SPAD and an IR SPAD can include a shallow-trench-isolation (STI)guard ring structure to reduce jitter and to reduce or eliminate adiffusion tail. An area-efficient shallow-trench-isolation guard ringstructure can prevent lateral drift and diffusion of charge carriers toenhance the detector's timing resolution. Certain technical informationfor silicon STI-SPADS is included in PCT Application No.PCT/US2007/074057 entitled “Shallow-Trench-Isolation (STI)-BoundedSingle-Photon Avalanche Photodetectors” and published as PCT publicationNo. WO 2008011617, which is incorporated by reference in its entirety aspart of the disclosure of this document. The trench isolation guard ringis capable of withstanding high electric fields and encloses a boundaryof a p-n junction region to spatially confine diffusion of charges atthe p-n junction to planarize the interface of the p-n junction of thediode. As a result, the guard ring can be used to prevent prematurebreakdown and to enhance uniformity of the electric filed distributionalong the pn junction and thus the detection probability across thedetection area or active area of the p-n junction. A guard ring can beformed to enclose the boundary of either or both of the shallow and deeppn junctions. Implementations of such guard rings can be used to achievehigh fill factors and small pixels in compact and high performancesensor arrays.

FIG. 8B shows an example of electric field distributions associated witha structure that includes a STI guard-ring structure. In FIG. 8B, theelectric field is given in volts per centimeter and coordinatemicrometers. In a detector device that includes an STI structure, aregion in the device that spatially corresponds to the lightly diffusedregion as shown in FIG. 8A can be replaced by SiO₂ to eliminate lateraldrift and to increase timing resolution. In some implementations, thetiming resolution of a STI-based CMOS SPAD can be 27-ps or less atfull-width at half-maximum, and the diffusion tail can exhibit 96-psfull-width at hundredth-maximum.

A STI-based CMOS SPAD fabrication process can include adeep-submicrometer CMOS process such as one based on 0.18-μm CMOStechnology. A STI's isolation trench can be constructed early in thefabrication process. In some cases, an isolation trench can preventpunch-through and latch-up in CMOS circuits. The edges of the subsequentdrain implant are confined by an oxide trench that prevents lateraldiffusion and formation of curved edges. In some implementations, such amanufacturing process can include using an IBM 0.18-μm CMOS technologythrough the MOSIS service, packaged in a high-speed quad flat no-lead(QFN) glass-top package with ultrashort wiring to minimize capacitanceand inductance.

In some implementations, two different output buffers can be used, e.g.,a source follower and an inverter chain. A source-follower buffer canuse a circuit that outputs a voltage proportional to the current flow toallow a direct observation of an avalanche pulse. An inverter chainoutput buffer can generate uniform-amplitude pulses for improvedcompatibility with pulse counting electronics.

FIG. 9A, 9B show instrument response function (IRF) examples fordifferent STI-bound SPAD devices with a source-follower output bufferbiased 7% above its breakdown voltage. FIG. 9A shows an IRF of STI-boundSPAD device with an active area of 2 μm×2 μm. FIG. 9B shows an IRF ofSTI-bound SPAD device with an active area of 14 μm×14 μm. These activeareas are characterized to determine the mechanism with the greatestimpact on the device jitter such as longitudinal diffusion or lateraldrift. Both devices were biased 7% beyond their breakdown voltage of 11V. The jitter of STI guard-ring SPAD with an 2 μm×2 μm can have a 26.7ps jitter at FWHM and a 96.1 ps jitter at FW( 1/100)M. The jitter of STIguard-ring SPAD with an 14 μm×14 μm. can have a 27.4 ps jitter at FWHMand a 98.9 ps jitter at FW( 1/100)M. Other dimensions for an active areaare possible. The similar pulse arrival time histograms of the 2- and14-μm devices demonstrate that lateral avalanche spreading is not thedominant jitter mechanism in these devices. The symmetrictime-of-arrival histogram is typical of avalanches seeded in thehigh-field region; the lack of a long diffusion tail indicates thatlateral drift is virtually eliminated. Longitudinal diffusion is alsoshown to be minimal.

FIG. 9C shows an IRF of STI-bound SPAD device with an active area of 7μm×7 μm with inverter output buffer biased at 7% beyond its breakdownvoltage. The STI-bound SPAD IRF exhibits a factor-of-two improvement inthe FW( 1/1000)M, from 1256 to 624 ps, when compared to a diffusedguard-ring SPAD with a match active area dimension. The longer IRF inFIG. 9C when compared to those in FIGS. 9A and 9B with source-followeroutput buffers are due to a suboptimal avalanche pulse sensingthreshold. The sensing threshold can be positioned low at the onset ofthe avalanche pulse. The source-follower output stage follows thediode's N-well terminal voltage, making it possible to search for anoptimal sensing threshold. The inverter stage can have a fixed thresholddefined by the relative sizes of its transistors. Consequently, itssensing threshold could not be tuned in the current test chip, and therespective IRF was longer.

In addition to the low jitter of the STI-bound SPAD device, such asdevice can exhibit shorter dead times and higher fill factor thandiffused guard-ring SPADs. This can come at the expense of relativelyhigh dark count rates (e.g., 104-106 counts/s) and afterpulsing,possibly due to interface states at the SiO₂—silicon boundary. Deviceimplementations can include time gating and active recharging to reducethe effects of this noise to acceptable levels. In some implementations,a STI-bound SPAD can be scaled to megapixel arrays to enable improvedperformance in optical communication channels and better temporalresolution in biological applications.

A detector device can include an array of detector pixels. In someimplementations, an array of III-V detector pixels can be constructedsuch that an avalanche region is spatially confined and an array of SiSPADs is aligned (e.g., using infrared imaging since it is transparentto Si) to the III-V detector pixels such that the active areas of the SiSPAD is aligned to the avalanche regions of the III-V semiconductorregion. In some implementations, a detector device can include a metallayer on the III-V semiconductor region to minimize inter-pixelcross-talk. In some implementations, a detector device can include ametal layer on the Si semiconductor region semiconductor region tominimize inter-pixel cross-talk. A detection system can include an arrayof detector pixels configured for low-light level infrared imaging.

A detector device can be configured for front side illumination or backside illumination. In a front side illumination example, IR photonsimpinge on the back surface (bulk side) of the silicon wafer, passthrough it and the SiO₂ dielectric (since Si is transparent to IRphotons) and is absorbed by the III-V junction. In a back sideillumination scheme, the III-V junction can be formed on top of anIR-transparent substrate. IR photons impinge on the back side of theIII-V detector.

Some photon detector devices can include a mechanism for self-quenchingand self-recovery within an InGaAs SPAD. At a critical field, the highprobability of impact ionization through a semiconductor can generate alarge avalanche current. The multiplication gain resulting from a singlecarrier undergoing this process can be in the millions, so a singlephotogenerated carrier can be detected. Left alone, this large avalanchecurrent can continue and, due to presence of the large avalanche currentcaused by the first photon, the photodetector cannot detect a secondphoton. Some InGaAs SPADs cannot self-quench or cannot self-recoverbecause they use an external circuit for quenching and recovery.However, such external circuitry can increase manufacturing costs andhave performance issues. This document includes details of SPADs, suchas an InGaAs SPAD, with a self-quenching and self-recovery mechanismwithout using the external quenching and recovery circuitry. Forexample, a SPAD can include a semiconductor layer in a detector stackthat functions as a negative feedback mechanism for self-quenching andself-recovery.

A self-quenching, self-recovering SPAD can be configured to producesecondary photon emission. In some implementations, a self quenchingavalanche photodiode can be made via bandgap engineering by using III-Vmaterials latticed matched to InP. A buffer region such as a TransitCarrier Buffer (TCB) can be positioned next to a multiplication regionto generate an energy barrier. Then the resulting electrons from theavalanche will be momentarily stopped by the barrier, e.g., TCB. Thisbarrier can reduce the field across the multiplication region and thusstops the avalanche process. When the avalanche process triggered by anincident photon is stopped, the device is self quenched. As theelectrons escape from the barrier, the field across the multiplicationregion recovers. At this time, the self-quenched detector is capable ofdetecting a second photon via another avalanche process.

The high energies of carriers undergoing avalanche breakdown can resultin photoemission with energies larger than the bandgap of the material.By utilizing this property, a separate absorption and multiplication APDcan detect low energy photons with a small bandgap absorption region,and generate photons in the multiplication region with higher energies.For example, an InGaAs—InP system can detect 1550 nm infrared, andoutput light such as visible light. In some implementations, themultiplication region is placed near the surface of the device tominimize self-absorption.

FIG. 10A shows an example of a device that includes a SPAD layout forself-quenching and self-recovery. A SPAD layout can include a a TCBregion 1010, and a multiplication region 1015, and an absorption region1020. Different SPAD layouts can include different layouts for a TCBregion 1010, and a multiplication region 1015, and an absorption region1020. In some implementations, a buffer region can be positioned beforeor after a multiplication region. In some SPAD layouts, a buffer regioncan be structured to impede electrons or holes from an avalanche processfrom passing through the buffer region to cause a temporary reduction inan electric field across a multiplication region to quench theavalanche.

FIG. 10B shows an example of a band diagram during self quenching andself recovery where the bottom shows the corresponding semiconductorregions for the band diagram. This example shows a valence band offsetto stop holes.

FIG. 11A, 11B, 11C show an example of a sequence of events in a SPADdetector device that includes a Transit Carrier Buffer. In this example,the detector device includes an absorption region 1101, a TransitCarrier Buffer (TCB) 1102, and a multiplication region 1103. Theabsorption region 1101 absorbs an incident photon 1105 and generates oneor more photongenerated carriers. The photongenerated hole 1110 in theabsorption region 1101 enters the multiplication region 1103 via the TCB1102 and can result in an avalanche generation of holes and electrons1115 in the multiplication region 1103. The recombination of the holesand electrons in the multiplication region 1103 produces high energyphotons (1120) as secondary photon emission. The electrons produced inthe avalanche process in the multiplication region can collect at theTCB barrier (1125) and this causes the electric field across themultiplication region 1103 to be lowered to stop the avalanche process.This process quenches the avalanche as indicated by the arrow 1130 inFIG. 11B. In turn, electrons collected at the barrier TCB escape thebarrier TCB and this condition recovers the device as indicated by thearrow 1135 in FIG. 11C for detecting another incident photon via theavalanche process.

In some implementations, a self-quenching SPAD can include a selfquenching layer, such as a TCB, designed by using the band offsets ofvarious epitaxial layers lattice matched to an InP substrate. A regionwith a high band offset, called a transit carrier buffer (TCB), isplaced next to a multiplication region. Carriers generated from theavalanche collect at the interface due to the band offset of theheterojunction between the TCB and the multiplication region, decreasingthe electric field through the multiplication region below breakdown ina short time (e.g., <0.1 ns), self-quenching the device. After theavalanche pulse, on a longer time scale (e.g., 10 to 100 ns), thecarriers escape from the barrier via thermal excitation and tunneling,self-recovering the device to a ready state for detecting anotherphoton.

A TCB can include a material with a valence band offset next to thep-layer of the multiplication p-i-n to stop holes. In someimplementations, a TCB can include material with a conduction bandoffset next to an n-layer of p-i-n semiconductor layers to stopelectrons.

In some implementations, by coupling a negative feedback mechanism withthe gain generated from the avalanche process, the current and voltageresponse of the device can then be simulated in Geiger mode operation,although the DC gain in this mode is infinite.

Referring to FIG. 10B for modeling the self-quenching based on thepresence of the TCB barrier between the absorption region at x<0 and themultiplication region, it is assumed that the multiplication region isof a thickness W, and the TCB quenching layer is of thickness L. Asingle hole is assumed to have reached the interface at x=0 between themultiplication region and the second photon absorption region. This holeis generated through photogeneration in the absorption region at x<0,and drifts to the edge of the multiplication region. Thermal generationwithin the multiplication region is considered negligible in thiscalculation due to the higher bandgap of the material.

Upon reaching x=0, additional carriers are generated through impactionization. A deterministic Selberherr's model is used, with impactionization dependent only on the local field near the carrier. Theionization rates for electrons and holes are then given by equations(13) and (14), respectively.α(E)=α₀exp(−[c _(n) /E] ^(m) ^(n) )  (13)β(E)=β₀exp(−[c _(p) /E] ^(m) ^(p) )  (14)The continuity equations in the multiplication region are

$\begin{matrix}{{e\frac{\partial\;}{\partial t}{p\left( {x,t} \right)}} = {{{- \frac{\partial}{\partial x}}{J_{p}(x)}} + {eG}_{p}}} & (15) \\{{{- e}\frac{\partial\;}{\partial t}{n\left( {x,t} \right)}} = {{{- \frac{\partial\;}{\partial x}}{J_{n}(x)}} - {eG}_{n}}} & (16)\end{matrix}$where the generation rates are G_(p)=G_(n)=βJ_(p)+αJ_(n). The current inthe multiplication region is then given by

$\begin{matrix}{{{\frac{1}{v}\frac{\partial\;}{\partial t}{J_{p}\left( {x,t} \right)}} + {\frac{\partial\;}{\partial x}{J_{p}\left( {x,t} \right)}}} = {{\beta\; J_{p}} + {\alpha\; J_{n}}}} & (17) \\{{{\frac{1}{v}\frac{\partial\;}{\partial t}{J_{n}\left( {x,t} \right)}} + {\frac{\partial\;}{\partial x}{J_{n}\left( {x,t} \right)}}} = {{\beta\; J_{p}} + {\alpha\; J_{n}}}} & (18)\end{matrix}$where it is assumed the carriers travel at their saturation velocity dueto the high field. The total current is the summation of the particlecurrent and the displacement current:

$\begin{matrix}{J = {{J_{p}(x)} + {J_{n}(x)} + {\frac{ɛ}{W}\frac{\mathbb{d}V_{m}}{\mathbb{d}t}}}} & (19)\end{matrix}$where V_(m) is the voltage across the multiplication region. Since thecurrent J is independent of x, the following equation is derived:

$\begin{matrix}{{\frac{\partial\;}{\partial x}J} = {0 = {{\frac{\partial\;}{\partial x}{J_{p}(x)}} + {\frac{\partial\;}{\partial x}{J_{n}(x)}}}}} & (20)\end{matrix}$

The voltage applied across the device is assumed to drop only due to themultiplication and TCB regions. Then the voltage across the TCB regionis V_(TCB)=V−V_(m). As the avalanche process builds up, holes aretrapped at the TCB interface due to the band discontinuity, forming asheet charge Q_(i). The TCB region can be modeled as a leaky capacitor,e.g., a capacitor and resistor in parallel. This would have a timeconstant τ, which represents the escape time of holes from theinterface. The current through the TCB region is then

$\begin{matrix}{J = {{\frac{ɛ}{L\;\tau}\left( {V - V_{m}} \right)} + {\frac{ɛ}{L}\frac{\mathbb{d}\left( {V - V_{m}} \right)}{\mathbb{d}t}}}} & (21)\end{matrix}$Using the boundary condition of zero electrons at the end of themultiplication region (e.g., at x=W), with the definitions T=τ(1+L/W),A=β−α, and ν=2(ν_(p)+ν_(n)), the above equations can be rearranged into

$\begin{matrix}{{{\frac{{\mathbb{e}}^{AW} - 1}{A}\frac{T}{v}\frac{\mathbb{d}^{2}V_{TCB}}{\mathbb{d}t^{2}}} + {\left( {{T\left\lbrack {1 - {\left( {{\mathbb{e}}^{AW} - 1} \right)\frac{\alpha}{A}}} \right\rbrack} + \frac{{\mathbb{e}}^{AW} - 1}{Av}} \right)\frac{\mathbb{d}V_{TCB}}{\mathbb{d}t}} + {\left\lbrack {1 - {\left( {{\mathbb{e}}^{AW} - 1} \right)\frac{\alpha}{A}}} \right\rbrack V_{TCB}}} = {\frac{L\;\tau}{ɛ}{\mathbb{e}}^{AW}{J_{p}(0)}}} & (22)\end{matrix}$

A numerical solution to equation (22) can be calculated with boundaryconditions V_(m)=V_(applied) and dV_(m)/dt=0 for t<0. Given an effectivecross sectional area S (typically about 100 μm²), a single photonresponse can be modeled byJ _(p)(0,t)=δ(t)e/S  (23)Unless noted otherwise, calculations are made at a 5% relative overbias,e.g., 105% of the breakdown voltage. The breakdown voltage is determinedby setting the multiplication factor to infinity in equation (24), wherek=β/α and the ionization coefficients are assumed to be positionindependent in the p-i-n junction.

$\begin{matrix}{M = \frac{1 - k}{{\exp\left( {{- \alpha}\;{W\left( {1 - k} \right)}} \right)} - k}} & (24)\end{matrix}$

FIGS. 12A and 12B show different response profile examples of aself-quenching, self-recovery SPAD with a structure shown in FIGS. 10Aand 10B. FIG. 12A shows a calculated current response due to a singlephoton at time=100 ns. FIG. 12B shows a voltage response across themultiplication layer of the SPAD. The long discharge time suggests amuch longer self-recovery time than the self-quenching time. Biasedabove the breakdown voltage, a single hole arriving in themultiplication region will trigger the avalanche. The interface chargerapidly accumulates at the TCB, dropping the voltage across themultiplication region, leading to self-quenching of the device. Theself-quenching mechanism is dependent on the avalanche build up processand occurs on a very short timescale (e.g., in the order 100 ps). Afterself-quenching, the avalanche current drops quickly, and the currentafter the avalanche pulse is due to the discharging of the accumulatedinterface charge. As the interface charge escapes, the voltage acrossthe TCB region decreases, or the voltage across the multiplicationregion increases, and the device self-recovers. The self-recoverymechanism is dependent on the hole escape process and occurs on a muchlonger timescale (>10 ns).

FIGS. 13A, 13B, 13C, 13D show different graphs associated with aself-quenching, self-recovery SPAD. FIG. 13A shows an example of a graphfeaturing gain versus overbias. FIG. 13B shows an example of a graphfeaturing quenching time versus overbias. FIG. 13C shows an example of agraph featuring recovery time versus overbias. FIG. 13D shows an exampleof a graph featuring average dark current level versus applied voltageat various input rates.

The dependence of several device characteristics on the applied bias canbe found from the current response of a single photon. The gain iscalculated by integrating the current under the avalanche pulse, and isshown to have a linear relationship to the applied bias (e.g., see FIG.13A). The quenching time is approximated by the full width at halfmaximum (FWHM) of the avalanche pulse, and is inversely proportional tothe applied overbias (e.g., see FIG. 13B). As the applied biasincreases, the buildup of interface charges increases faster, thusleading to the faster quenching time. The self-recovery time is definedas the time when a second input will produce a gain of 50% of the firstinput (e.g., see FIG. 13C). In some implementations, there can be aslight dependence on the applied voltage due to the dependence of theinitial gain on the applied voltage; however, this dependence can beweak, and the recovery time can be limited by the intrinsic RC.

The current response to an input photon is strongly dependent on thevoltage across the multiplication region. If a secondary photon isintroduced before full recovery, the resulting current gain may be lessthan ideal, and thus the device can have a maximum detection ratedepending on the output sensitivity needed.

FIGS. 14A, 14B, 14C, and 14D show response examples of a self-quenchingand self-recovering SPAD to multiple input photons at various rates.FIG. 14A shows an example of a calculated voltage response due to aphoton input rate of 10 MHz. FIG. 14B shows an example of a calculatedvoltage response due to a photon input rate of 50 MHz. FIG. 14C shows anexample of a current response due to a photon input rate of 10 MHz. FIG.14D shows an example of a current response due to a photon input rate of50 MHz. Higher input rates lead to less time for recovery, resulting inlower peak current as demonstrated by the figure. Note that at time=120ns for the 50 MHz (e.g., 5×107 photons/s or 5×107 dark counts/s) case,the voltage across the multiplication region is still below thebreakdown voltage, and thus there is little response.

This same setup can also be used to model the dark current levels of thedevice. If it is assumed dark carriers are generated at a given rate,the dependence of the average current level on the applied bias can beplotted (e.g., see FIG. 13D). While at low rates the average currentlevel is directly proportional to the dark current rate, at high darkcurrent rates the relationship is no longer linear. This is due todevice saturation, and each additional input carrier produces littlegain.

To optimize device characteristics such as gain and recovery time, thevariation of the device structure and material parameters is examined.The hole escape time τ, which physically depends on barrier height andthickness, along with the TCB layer thickness L is varied. Longer holeescape times lead to a longer self-recovery time of the device. In someimplementations, shorter escape times mean there will be fewer holestrapped at the interface, leading to a longer quenching time. As thedevice takes longer to quench, the device can remain in a Geiger modefor a longer time, and thus produces a larger gain. FIG. 15A shows agraph of a relationship between Geiger-mode gain and hole escape time.

FIG. 15B shows an example of a relationship between Geiger-mode gain andTCB layer thickness. When the barrier thickness is smaller, its modeledcapacitance increases, and thus more charge is required to quench theavalanche, resulting in a larger Geiger-mode gain. In someimplementations, varying either L or τ can have similar effects (e.g.,see equation (22)).

The modeling of the SPAD with negative feedback shows gain can beincreased by increasing the applied overbias, at the cost of increasingdark current. Increasing the applied bias will also benefit keycharacteristics such as the quenching time. To maximize gain, either theself-quenching barrier height (related to hole escape time) or barrierthickness can be modified. As either is decreased, the infinite gain ofa conventional SPAD in Geiger mode is recovered; but doing so diminishesthe fast self-quenching property. As gain is much more sensitive to thebarrier thickness, a thin and high barrier is preferred to a low andthick barrier.

A detector can include an InGaAs/InAlAs based SPAD to detect singlephotons at λ=1550 nm, with a quenching layer InGaAsP (E_(g)=1 μm). Insuch a detector, the resulting valence band offset between an InAlAsmultiplication region and a TCB region is around 80 meV.

FIG. 16 shows example of an I-V response of a self-quenching andself-recovering device in the dark and in 1550 nm illumination. Such adevice can include an InGaAs/InAlAs based SPAD to detect single photonsat λ=1550 nm. The device can include a quenching layer InGaAsP (E_(g)=1μm). In such a device, the resulting valence band offset between anInAlAs multiplication region and a TCB region is around 80 meV. Due tothe presence of the TCB layer, the current does not increase as rapidlynear the breakdown voltage compared to conventional SPADs.

The avalanche pulse width is found to be around 30 ns (limited by theelectronics rather than the intrinsic response), with a device recoverytime of around 300 ns. Due to the self-quenching mechanism, theavalanche pulses can be uniform.

FIG. 17A shows an example of an output pulse height at a bias voltage of30.4 V. FIG. 17B shows an example of output pulse signals triggered by aseries of single photons. As seen in FIGS. 17B and 17B, the averagepulse height is 0.76 V, with a standard deviation of 25 meV. Theequivalent excess noise factor calculated from the pulse height is1.001.

FIG. 18 show an example of different single photon detectionefficiencies versus dark current rates at various temperatures. Thedecrease in single photon detection efficiencies (SPDE) from 160 K to120 K is due to the large decrease in the absorption coefficient at 1550nm wavelength for InGaAs. The decrease in SPDE per given dark currentrates (DCR) at higher temperatures can be attributed to less time forrecovery between events, leading to more holes still trapped at theinterface. This decreases the effective voltage across themultiplication region, leading to smaller gains and SPDE.

FIG. 19 shows an example of a recovery time profile. The self-recoverytime can also be deduced from the correlation measurements. After atriggered pulse, at time intervals less than the recovery time the darkcount will be significantly less. The recovery time is defined here asthe time at which the avalanche probability recovers to 63% of itsnormal level. As the reverse bias increases, the device recovery timedecreases, which suggests that the hole escaping mechanism for this TCBstructure is dominated by field assisted tunneling. Additionalmeasurements from 120 K to 240 K show that the temperature dependence ofthe self-recovery time is weak.

FIG. 20 shows a cross-section example of a SPAD in a detector device.Such a device can include separate absorption and multiplicationstructure (SAM), where absorption occurs in an InGaAs layer andmultiplication is performed in an InAlAs layer. Such a device caninclude additional layers for lattice matching and for additionalfunctionality. Different implementations can have more or less layersthan those shown in FIG. 20. Different implementations can havedifferent layers than those shown in FIG. 20.

In the SPAD design in FIGS. 10A and 10B, the bandgap of themultiplication region 1015 and the bandgap of the absorption region 1020for absorbing photon emitted by the multiplication region 1015 aredifferent. At the interface between the regions 1020 and 1015, thebandgap has a step when transitioning between the regions 1010 and 1015.This abrupt step change in the bandgap at the interface reduces theefficiency of the SPAD. One technique to mitigate the reduced detectionefficiency caused by the abrupt step change in the bandgap is insertinga semiconductor transition structure that has a spatially graded bandgapbetween the regions 1020 and 1015. At the interface between themultiplication region 1015 and the first side of the semiconductortransition structure, the semiconductor transition structure exhibits abandgap that is equal to or similar to the bandgap of the multiplicationregion 1015. At the other side of the semiconductor transition structureis the interface with the absorption region 1020 and the bandgap of thesemiconductor transition structure at this interface is equal to orsimilar to the bandgap of the absorption region 1020. From the firstside to the second side of the semiconductor transition structure, thebandgap spatially varies in a monotonic manner to form a spatiallyvarying ramp. This spatially varying ramp in the bandgap can improve thedetection efficiency in comparison with a similarly constructed SPADdevice without the semiconductor transition structure.

In some implementations, a detector device can include a semiconductorabsorption region structured to absorb photons at a first wavelength togenerate one or more charged carriers. The detector device can include amultiplication region structured to receive the one or more chargedcarriers, the multiplication region structured to generate an avalancheof electrons in response to the one or more charged carriers and emitsecondary photons at a second wavelength shorter than the firstwavelength. The detector device can include a buffer region structuredto impede electrons or holes from the avalanche from passing through thebuffer region to cause a reduction in an electric field across themultiplication region to quench the avalanche. The detector device caninclude a bandgap grading region adjacent to the absorption region, atleast a portion of the bandgap grading region having a spatially varyingbandgap profile that monotonically changes between a first region thatinterfaces with the absorption region and a second region.

FIG. 21A shows an example of a SPAD detector device 2100 that includesthe semiconductor transition structure made of one or more bandgapgrading regions and sandwiched between the multiplication and theabsorption region. The detector device 2100 includes an absorptionregion 2130, a transit carrier buffer region 2115, and a multiplicationregion 2125. A detector device 2100 includes one or more bandgap gradingregions 2135. A bandgap grading region 2135 is designed to exhibit aspatially varying bandgap that changes between successive layers of thebandgap grading region 2135. For example, bandgap grading region 2135can have a spatially varying bandgap profile that monotonically changesbetween a first region that interfaces with a multiplication region 2125and a second region that interfaces with the second absorption region2130.

FIG. 21B shows an example of an InGaAs/InAlAs SPAD with a built-inquenching mechanism based on the quenching layers and a transitionstructure between the multiplication region and the absorption region.In this example, the epilayer structure of the self-quenching SPADincludes an buffer region (e.g., multiple quenching layers),multiplication region, a transition region formed of bandgap gradinglayers and an absorption region. The absorption region includes a 1.5 μmthick InGaAs layer. A multiplication region includes InAlAs p-i-njunction with a 0.2 μm intrinsic region. The bandgap grading layers aregrown between an InGaAs absorption region and an InAlAs multiplicationregion to provide the spatially varying bandgap ramp from the bandgap ofthe multiplication region and the absorption region. As an example,different bandgap grading layers have different bandgaps thatsequentially and monotonically change from one side of the bandgapgrading layers to the other side. A quenching layer can include InGaAsPwith E_(g)=1.0 μm. An energy barrier is created by the valence bandoffset between InGaAsP with E_(g)=1.0 μm and InAlAs, which is, forexample, 80 meV.

FIGS. 21C and 21D show different bandgap grading region layouts. Asshown in FIG. 21C, a detector device layout can position a bandgapgrading region 2170 positioned between a multiplication region 2165 andan absorption region 2175. As shown in FIG. 21D, a detector devicelayout can position a bandgap grading region 2185 between a bufferregion 2180 and an absorption region 2190.

Different TCB structures and arrangements can be implemented. Table 1and Table 2 show different example of detector device structures.

Table 1 shows an example of an epitaxial layer structure of aself-quenching and self-recovering device. An InAlAs layer is placed inbetween the InP multiplication and InGaAs absorption regions to act as aTCB for electrons. Graded transition layers are placed in between tosupport the transport of photogenerated holes from InGaAs to InP. Thesystem can be capped with a thin p layer for an ohmic contact.

TABLE 1 Material Thickness Doping Function InGaAsP 0.1 μm p = 1e18 Ohmiccontact InP 0.2 μm p = 2e17 Multiplication region InP 0.8 μmMultiplication region InP 0.25 μm  n = 1e17 Multiplication regionInGaAsP GRIN 0.1 μm Transition layer (1.1 um to InP) InAlAs 0.8 μm TCBInAlGaAs GRIN 0.3 μm Transition layer InGaAs 1.5 μm Absorption layer InPbuffer 0.5 μm n = 1e17 InP n substrate

TABLE 2 Material Thickness Doping Function InGaAsP 0.1 μm p = 1e18 Ohmiccontact InP 0.2 μm p = 2e17 Multiplication region InP 0.8 μmMultiplication region InGaAsP GRIN 0.1 μm Transition layer (1.1 um toInP) InAlAs 0.3 μm n = 7e16 TCB InGaAsP (1.1 um) 0.2 μm InAlAs 0.3 μmTCB InAlGaAs GRIN 0.15 μm  Transition layer InGaAs 1.5 μm Absorptionlayer InP buffer 0.5 μm n = 1e17 InP n substrate

Table 2 shows a different example of an epitaxial layer structure of aself-quenching and self-recovering device. In some implementations, adevice can include two or more TCB layers. In some implementations, thefield drop can occur across the first TCB barrier.

FIGS. 22A and 22B show different implementation examples of anInGaAs/InAlAs SPAD with a built-in quenching mechanism. FIG. 22A showsan example of the SPAD's mesa structure in a top illuminationconfiguration where the top Au—Ti electrode is formed to cover the mesadetector structure while leaving an aperture on the top of the mesa forreceiving incident light. On the backside of the detector, a Ge/Auelectrode is formed. In operation, a control voltage is applied acrossthe top and backside electrodes to operate the detector. In someimplementations, the mesa sidewall is protected by a polyimidepassivation layer. FIG. 22B shows a scanning electron microscopy (SEM)micrograph of a fabricated SPAD device. In some implementations, theSPAD can operate in a free-running mode for single photon detectionwithout the need of external quenching circuit.

A detector device can include an InGaAs/InAlAs SPAD for near infraredlight detection with build-in self-quenching and self-recoverycapabilities. The device can detect light signal down to single photonlevel at near infrared wavelength. In some implementations, the detectordevice integrates four functions, photon absorption, avalanchemultiplication, avalanche quenching, and device resetting. The build-inself-quenching self-recovery capabilities can enable the device to beoperated with a simpler read-out circuit and in sub-Geiger mode, whichis in contrast to conventional SPADs Geiger mode operation. Insub-Geiger mode operation, the device is biased at DC voltage. In someimplementations, a detector device can include a mechanism, such anelectronic circuit, to operate the InGaAs/InAlAs SPAD in a DC conditionfor sub-Geiger operations. Biasing the detector device at DC voltage canreduce detection system complexity and can minimize the SPAD's deadtime. The detector device can achieve high multiplication gain with verylow excess noise. The detector device can be configured to count photonsfor photon number resolving applications. The detector device can have astable gain versus bias behavior.

An InGaAs/InAlAs SPAD can exhibit a gain saturation behavior, e.g., asincreasing the bias, the gain can increase to a certain value, (˜10⁵ to10⁶), and become saturated. The saturation gain value can be tuned byadjusting the energy barrier height and the quenching layer capacitance.A detection system can use a SPAD with this gain saturation property forincreased reliability, and for a lower voltage supply requirement,which, in turn can reduce system cost.

An InGaAs/InAlAs SPAD based on the techniques described in this documentcan be configured for self-quenching. A self-quenching process canregulate an avalanche multiplication process, in a multiplication regionof the SPAD, to achieve high gain (e.g., 10⁶) for single photondetection and in the same time suppress noise to a low level.

In some implementations, a detector device for single photon frequencyup-conversion can include an IR SPAD optically coupled with a CMOS SPAD.The photons produced by hot-carrier recombination process in the IR SPADcan be subsequently sensed by the CMOS SPAD to allow for on-die dataprocessing. In some implementations, a CMOS SPAD can detectup-converted, visible photons and can include an on-chip readoutcircuitry to process the signal. Coupling between the IR and CMOS SPADcan be accomplished using mature wafer-level glass-to-glass fusingtechnology to increase manufacturing efficiency and to reduce costs whencompared to other hetero-integration approaches. In someimplementations, the IR SPAD and CMOS SPAD can be fused through asilicon dioxide passivation layer. To achieve up conversion in thedetector device, the device can utilize a byproduct of the avalancheprocess, specifically the spectral component of the electroluminescentphotons which is higher than the bandgap of the absorbing material ofthe detector. The detector device can exhibit low noise, can have ashort dead time, can have superior up-conversion efficiencies with lowpower and can be scalable to large arrays.

FIG. 23 shows a different cross-section example of a device based on ahot-carrier luminescence effect. A detector device 2200 can include anIR absorption structure 2205, a multiplication structure 2210, one ormore dielectric layers 2215, 2220, a silicon secondary photon absorptionstructure 2225, CMOS circuitry 2230, and a silicon substrate 2235. Asilicon secondary photon absorption structure 2225 can include CMOSdetection and processing circuitry. In some implementations, a siliconsecondary photon absorption structure 2225 can include CMOS circuitry2230. In some implementations, the device can include one or moreisolation structures 2250.

Detector device 2200 can be structured to provide the absorption of IRand multiplication of the carriers to produce light at an opticalwavelength shorter than the IR light. The absorption material can be anarrow bandgap IR absorption material and thus can absorb in the IRspectral range. The multiplication material can have a wide bandgapgreater than the narrow bandgap to reduce noise. The electroluminescencepeaks near the bandgap of the multiplication material's bandgap and thushas higher energy than the absorbed photon. It also has a tail with evenhigher energies, due to the energy distribution of hot carriers. This ishow the upconversion is achieved. Even in materials with the sameabsorption and multiplication materials, upconversion is possible, dueto the above-mentioned tail, but in this case it will be less efficient.Also, if the multiplication material is a direct bandgap material,electroluminescence through hot-carrier recombination can be moreefficient.

FIG. 24 shows an example of a detector architecture. A detector device2400 can include a first optical sensing structure 2405, a secondoptical sensing structure 2410, and circuitry such as counting circuitry2415. In some implementations, one or more optical structures 2405, 2410can be mounted on a cryostat. Counting circuitry 2415 can receive anoutput signal from the second optical sensing structure 2410. Countingcircuitry 2415 can include a blocking capacitor, amplifier, and a pulsecounter. For example, an output signal from the second optical sensingstructure 2410 can pass through a blocking capacitor to an amplifierwith a voltage gain of, for example, 200 and a bandwidth of, forexample, 1 GHz. An output of the amplifier can be connected with a pulsecounter to count pulses that correspond to a detected photon.

While this document contains many specifics, these should not beconstrued as limitations on the scope of an invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis document in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or a variation of a subcombination.

Only a few implementations are disclosed. However, variations andenhancements of the described implementations and other implementationscan be made based on what is described and illustrated.

What is claimed is:
 1. A semiconductor radiation sensing device,comprising: a semiconductor absorption region structured to absorbphotons at a first wavelength to generate one or more charged carriers;a multiplication region structured to receive the one or more chargedcarriers, the multiplication region structured to generate an avalancheof electrons in response to the one or more charged carriers and emitsecondary photons at a second wavelength shorter than the firstwavelength; a buffer region structured to impede electrons or holes fromthe avalanche from passing through the buffer region to cause areduction in an electric field across the multiplication region toquench the avalanche; and a bandgap grading region adjacent to theabsorption region, at least a portion of the bandgap grading regionhaving a spatially varying bandgap profile that monotonically changesbetween a first region that interfaces with the absorption region and asecond region.
 2. The device as in claim 1, wherein the buffer region isstructured to allow electrons to pass through the buffer region to causean increase in the electric field across the multiplication region tofacilitate a recovery from the avalanche.
 3. The device as in claim 1,wherein the buffer region is structured to allow holes to pass throughthe buffer region to cause an increase in the electric field across themultiplication region to facilitate a recovery from the avalanche. 4.The device as in claim 1, wherein the bandgap grading region ispositioned between the absorption region and the multiplication region,wherein the second region of the bandgap grading region interfaces withthe multiplication region.
 5. The device as in claim 1, wherein thebandgap grading region is positioned between the absorption region andthe buffer region, wherein the second region of the bandgap gradingregion interfaces with the buffer region.
 6. The device as in claim 1,wherein the buffer region and the multiplication region are structuredto create an energy barrier by a valence band offset.
 7. The device asin claim 1, further comprising: a mechanism to bias the second opticalsensing structure at a DC voltage.
 8. The device as in claim 1, whereinthe multiplication region is optically coupled with an optical sensingstructure.
 9. The device as in claim 1, wherein the buffer regioncomprises an InAlAs layer.
 10. The device as in claim 1, wherein thebuffer region comprises an InAlAs, InGaAsP, and InAlAs stack.
 11. Thedevice as in claim 1, wherein the bandgap grading region comprises oneor more InGaAsP graded index layers.
 12. A semiconductor radiationsensing device, comprising: a semiconductor absorption region structuredto receive light at a first wavelength to generate one or more chargedcarriers by absorbing received light; a multiplication region structuredto receive the one or more charged carriers generated from thesemiconductor absorption region and to generate an avalanche ofsecondary charged carriers in response to the one or more chargedcarriers and emit secondary photons from the secondary charged carriers;a region coupled between the absorption region and the multiplicationregion, the region comprising a mechanism that quenches the avalanche ofthe multiplication region after occurrence of the avalanche and resetsthe multiplication region for a next avalanche; and a semiconductortransition region formed between the multiplication region and theabsorption region to have a first bandgap at a first interface with themultiplication region that is equal to or similar to a bandgap of themultiplication region and a second bandgap at a second interface withthe absorption region that is equal to or similar to a bandgap of theabsorption region, the semiconductor transition region having aspatially varying bandgap between the first and second interfaces toeliminate an abrupt change in bandgap between the multiplication regionand the absorption region.
 13. The device as in claim 12, wherein thesemiconductor transition region comprises a plurality of semiconductorlayers that have different bandgaps to form the spatially varyingbandgap between the first and second interfaces.
 14. A semiconductorradiation sensing device, comprising: a semiconductor absorption regionstructured to receive light at a first wavelength to generate one ormore charged carriers by absorbing received light; a multiplicationregion structured to receive the one or more charged carriers generatedfrom the semiconductor absorption region and to generate an avalanche ofsecondary charged carriers in response to the one or more chargedcarriers and emit secondary photons from the secondary charged carriers;a region coupled between the absorption region and the multiplicationregion, the region comprising a mechanism that quenches the avalanche ofthe multiplication region after occurrence of the avalanche and resetsthe multiplication region for a next avalanche; and a semiconductortransition region formed between the buffer region and the absorptionregion to have a first bandgap at a first interface with the bufferregion that is equal to or similar to a bandgap of the buffer region anda second bandgap at a second interface with the absorption region thatis equal to or similar to a bandgap of the absorption region, thesemiconductor transition region having a spatially varying bandgapbetween the first and second interfaces to eliminate an abrupt change inbandgap between the buffer region and the absorption region.
 15. Thedevice as in claim 14, wherein the semiconductor transition regioncomprises a plurality of semiconductor layers that have differentbandgaps to form the spatially varying bandgap between the first andsecond interfaces.
 16. A semiconductor device, comprising: a firstoptical sensing structure structured to absorb light at a first opticalwavelength; and a second optical sensing structure engaged with thefirst optical sensing structure to allow optical communication betweenthe first and the second optical sensing structures, wherein the secondoptical sensing structure is structured to absorb light at a secondoptical wavelength longer than the first optical wavelength and to emitlight at the first optical wavelength which is absorbed by the firstoptical sensing structure, wherein the first optical sensing structurecomprises a silicon substrate and is a silicon-based optical detector,wherein the second optical sensing structure is an IR optical detectorwhich emits light at the first optical wavelength by converting energyin absorbed light at the second optical wavelength via luminescenceresulting from hot carrier recombination.
 17. The device as in claim 16,further comprising: a silicon dielectric layer as an interfacing layerformed between the first optical sensing structure and the secondoptical sensing structure.
 18. The device as in claim 16, furthercomprising: a complementary metal-oxide-semiconductor (CMOS) circuit tocontrol the first optical sensing structure and the second opticalsensing structure.
 19. The device as in claim 16, wherein the secondoptical sensing structure comprises an absorption structure whichabsorbs light at the second optical wavelength and a multiplicationmaterial layer between the absorption structure and the first opticalsensing structure to emit light at the first optical wavelength.
 20. Thedevice as in claim 19, wherein the absorption structure has a bandgapless than a bandgap of the multiplication structure.
 21. The device asin claim 19, wherein the absorption structure has a bandgap similar to abandgap of the multiplication structure.
 22. A semiconductor device,comprising: a first optical sensing structure structured to absorb lightat a first optical wavelength; and a second optical sensing structureengaged with the first optical sensing structure to allow opticalcommunication between the first and the second optical sensingstructures, wherein the second optical sensing structure is structuredto absorb light at a second optical wavelength longer than the firstoptical wavelength and to emit light at the first optical wavelengthwhich is absorbed by the first optical sensing structure, wherein thesecond optical sensing structure comprises an absorption structure whichabsorbs light at the second optical wavelength and a multiplicationstructure between the absorption structure and the first optical sensingstructure to emit light at the first optical wavelength.
 23. The deviceas in claim 22, wherein the absorption structure has a bandgap less thana bandgap of the multiplication structure.
 24. The device as in claim22, wherein the absorption structure has a bandgap similar to a bandgapof the multiplication structure.
 25. A semiconductor device, comprising:a first optical sensing structure structured to absorb light at a firstoptical wavelength; a second optical sensing structure engaged with thefirst optical sensing structure to allow optical communication betweenthe first and the second optical sensing structures, wherein the secondoptical sensing structure is structured to absorb light at a secondoptical wavelength longer than the first optical wavelength and to emitlight at the first optical wavelength which is absorbed by the firstoptical sensing structure, and a dielectric layer interfacing betweenthe first and the second optical sensing structures to permittransmission of light and to fuse the first and the second opticalsensing structures together as a single structure.
 26. A semiconductordevice, comprising: a first optical sensing structure structured toabsorb light at a first optical wavelength; and a second optical sensingstructure engaged with the first optical sensing structure to allowoptical communication between the first and the second optical sensingstructures, wherein the second optical sensing structure is structuredto absorb light at a second optical wavelength longer than the firstoptical wavelength and to emit light at the first optical wavelengthwhich is absorbed by the first optical sensing structure, wherein a deadtime of the second optical sensing structure overlaps with an avalanchetime of the first optical sensing structure to prevent a positivefeedback loop between the first and second optical sensing structures.27. A semiconductor device, comprising: a first optical sensingstructure structured to absorb light at a first optical wavelength; anda second optical sensing structure engaged with the first opticalsensing structure to allow optical communication between the first andthe second optical sensing structures, wherein the second opticalsensing structure is structured to absorb light at a second opticalwavelength longer than the first optical wavelength and to emit light atthe first optical wavelength which is absorbed by the first opticalsensing structure, wherein the second optical sensing structurecomprises: an absorption region structured to absorb photons at thesecond wavelength, a multiplication region structured to generate anavalanche of electrons in response to an absorbed photon to causephotons to be emitted at the first optical wavelength, and a bufferregion coupled with the multiplication region, and structured to impedeelectrons or holes from the avalanche from passing through the bufferregion to cause a reduction in an electric field across themultiplication region to quench the avalanche, and to allow electrons orholes to pass through the buffer region to cause an increase in theelectric field across the multiplication region to facilitate a recoveryfrom the avalanche.
 28. The device as in claim 27, further comprising: abandgap grading region coupled with the multiplication region, at leasta portion of the bandgap grading region having a spatially varyingbandgap profile that monotonically changes between a first region thatinterfaces with the multiplication region and a second region.
 29. Thedevice as in claim 27, further comprising: a bandgap grading regioncoupled with the buffer region, at least a portion of the bandgapgrading region having a spatially varying bandgap profile thatmonotonically changes between a first region that interfaces with thebuffer region and a second region.
 30. The device as in claim 27,further comprising: a mechanism to bias the second optical sensingstructure at a DC voltage.
 31. The device as in claim 27, wherein themultiplication region is optically coupled with the first opticalsensing structure.
 32. The device as in claim 27, wherein the bufferregion comprises an InAlAs layer.
 33. The device as in claim 27, whereinthe buffer region comprises an InAlAs, InGaAsP, and InAlAs stack.
 34. Asemiconductor device, comprising: a first optical sensing structurestructured to absorb light at a first optical wavelength; and a secondoptical sensing structure engaged with the first optical sensingstructure to allow optical communication between the first and thesecond optical sensing structures, wherein the second optical sensingstructure is structured to absorb light at a second optical wavelengthlonger than the first optical wavelength and to emit light at the firstoptical wavelength which is absorbed by the first optical sensingstructure, wherein the first optical sensing structure comprises anarray of detector pixels, wherein the second optical sensing structurecomprises an array of detector pixels, the device further comprising oneor more layers to minimize inter-pixel cross-talk between the first andsecond optical sensing structures.